Don't install pytest-stress in python2 environment (sonic-net#21750)

The pytest-stress package is only compatible with Python 3. Trying to install it with Python 2 leads to a build failure for the sonic-mgmt docker image.

Signed-off-by: Lawrence Lee <lawlee@microsoft.com>

Author: theasianpianist

dockers/docker-sonic-mgmt/Dockerfile.j2

@@ -176,7 +176,6 @@ RUN python2 -m pip install allure-pytest==2.8.22 \
                 pytest-repeat \
                 pytest-html \
                 pytest-xdist==1.28.0 \
-                pytest-stress \
                 python-dateutil \
                 PyYAML \
                 redis \